1. Field of the Invention
The present invention relates to a projector that projects projection light on a projection object such as a screen to display an image, and in particular relates to a projector equipped with a zoom lens with which the size of the projection light region for projecting projection light can be changed, and an imager for imaging the projection light region.
2. Description of the Related Art
In recent years, a projector equipped with a zoom lens as the projection lens is known where the size of the projection light region formed on the screen can be freely varied by driving the zoom lens to change its zoom position.
Also, when such a projector is positioned in front of a screen, a variety of adjustments need to be made such as zoom adjustment, keystone correction, and focus adjustment on the projector in advance so the image is displayed correctly on the screen by the projection light projected by the projector onto the screen.
With portable projectors, however, the relative position to the screen may change each time the projector is set up, so the operator must make the above such adjustments each time, which is extremely troublesome.
As conventionally described in, for example, Japanese Patent Laid-Open Gazettes No. 2000-241874, when setting up a projector provided with a monitor camera in front of a screen, an adjustment pattern image is projected on the screen, the screen displaying the pattern image is imaged by the monitor camera, the imaged image is analyzed, and the above-mentioned adjustments are carried out automatically based on the results of the analysis.
In general, a monitor camera is provided with a charge-coupled device (CCD) or the like for converting incoming light to electrical signals, and has a function (auto-exposure) for varying the shutter speed, gain (sensitivity), aperture, and the like to make the overall light of the imaged image a set value (exposure target value).
FIG. 11 is an illustration depicting the effect of auto-exposure by a monitor camera of the prior art. At top in FIG. 11 are shown screens displaying adjustment pattern images; in the middle are shown imaged images obtained by imaging the screen with the monitor camera; and at bottom are shown values representing brightness of pixels lined up along a center horizontal line (broken line). (A) shows the condition with the projector in normal mode, and (B) shows the condition with the projector set to low luminance.
Values that represent pixel brightness in a imaged image shall herein be referred to as gradation values. Gradation values are values derived from an image signal of the imaged image output from the monitor camera (CCD module),
In FIG. 11, the adjustment pattern image is a completely white image; as shown at top, on the screen, the white portion area displayed as the pattern image is the projection light region mentioned previously.
The imaged image imaged by the monitor camera is as shown in the middle of the drawing.
At bottom in the drawing, the horizontal axis corresponds to pixel location along a center horizontal line through the imaged image, and the vertical axis gives pixel gradation values.
If the lamp which is the light source of the projector has been set to the low luminance setting, the luminance of the projection light projected from the projector will be lower as compared to that in normal operation. Thus, as shown in FIG. 11(B), the brightness of the pattern image displayed on the screen will be darker as compared to the normal operation depicted in (A). However, if the pattern image is imaged with auto-exposure by means of the monitor camera, despite the fact that the subject is dark, the shutter speed, gain, aperture and so on will be adjusted so as to give proper brightness to the imaged image overall, so that in the imaged image, pattern image brightness is no different from that in the normal operation of (A), as shown by FIG. 11(B). Accordingly, in the imaged image, the black portion (i.e. portion outside the projection light region) is dark enough to be ignored, and thus gradation values of pixels situated in the black portion are considered to be “0”, whereas thus gradation values of pixels situated in white portion (i.e. in pattern image portion) are substantially unchanged from the desired gradation values Lt.
By means of operating the auto-exposure function of the monitor camera in this way, even in instances where the light source lamp has been set to low luminance so that the pattern image displayed on the screen has low brightness, nonetheless in the imaged image, gradation values of pixels in the white portion (i.e. pattern image portion) can be substantially maintained at the desired gradation values Lt, just as during normal operation. This is true not only at the low luminance setting, but also where the light source lamp has declined in luminance due to degradation over time.
However, in a projector equipped with a zoom lens and a monitor camera, in cases where the zoom position of the zoom lens has been changed resulting in a change in the size of the projection light region on the screen, auto-exposure by the monitor camera results in problems such as the following.
FIG. 12 is an illustration depicting a problem caused by auto-exposure by a monitor camera in the case where zoom position has been changed. In FIG. 12, as in FIG. 11, at top are shown screens displaying adjustment pattern images; in the middle are shown imaged images of the screen; and at bottom are shown gradation values of pixels in the imaged images. (A) shows the condition where the zoom position of the zoom lens has been set to a medial position, (B) shows the condition where the zoom position has been set to the wide angle end, and (C) shows the condition where the zoom position has been set to the telephoto end.
Where the zoom position in the projector has been set to the wide angle end, as shown in FIG. 12(B), the area of the projection light region on the screen will be larger as compared to that medial position of (A), and pattern image displayed thereon will be enlarged as well. Accordingly, when this pattern image is imaged by the monitor camera, as shown at middle in FIG. 12(B), the area of the white portion (i.e. pattern image portion) in the imaged image will be larger as compared to that medial position of (A), and the black portion (i.e. portion outside the projection light region) will have smaller area.
When image is carried out by auto-exposure under these conditions, overall brightness of the imaged image is calculated as the calculated exposure value, whereupon shutter speed, gain, aperture and so on are controlled so as to make this calculated exposure value equal to the preset exposure target value. Here, overall brightness of a imaged image is the sum total of quantities derived by amplification of an electrical signal into which light detected at each pixel in the CCD has been converted; the value is proportional to the average value of gradation values of pixels in the imaged image. Thus, the calculated exposure value is typically the average value of gradation values of all pixels in the imaged image.
Since the exposure target value is a preset fixed value, if the calculated exposure value coincides with the exposure target value when the zoom position is at a medial position as shown in FIG. 12(A), in the event that the zoom position is then changed to the wide angle end so that the area of the white portion become larger, to the extent that these area are larger, the average value of gradation values of all pixels, i.e. the calculated exposure value, will be higher than the exposure target value. As a result, when the auto-exposure operates and changes the shutter speed, gain aperture and so on to make the calculated exposure value equal the exposure target value, the average value of gradation values of all pixels in the imaged image will be lower. As noted, in the imaged image, since the black portion is dark enough to be ignored and gradation values of pixels situated in the black portion are considered to be “0”, the fact that the average value of gradation values of all pixels is lower means that gradation values of pixels in the white portion will be lower than the desired gradation value Lt, as shown at bottom in FIG. 12(B).
Conversely, where the zoom position of the zoom lens is at the telephoto end, as shown at top in FIG. 12(C), the area of the projection light region on the screen will be smaller as compared to those at the medial position of (A), and the pattern image displayed thereon will be smaller as well. Accordingly, when the pattern image is imaged by the monitor camera, the area of white portion (i.e. pattern image portion) in the imaged image will be smaller as compared to those at the medial position of (A), and the area of the black portion (i.e. portion outside the projection light region) will be larger.
In this way, in instances where area of the white portion in a imaged image are smaller, the average value of gradation values of all pixels, i.e. the calculated exposure value, will be lower than the exposure target value. As a result, when the auto-exposure operates and changes the shutter speed, gain, aperture and so on to make the calculated exposure value equal the exposure target value, the average value of gradation values of all pixels in the imaged image will be higher, and as a result the gradation values of pixels in the white portion will be higher than the desired gradation value Lt, as shown at bottom in FIG. 12(C).
As described hereinabove, in the past, in the case where the zoom position of a zoom lens is set at the wide angle end so that the area of the projection light region on the screen are larger, auto-exposure by the monitor camera will result a imaged image whose gradation values of pixels in the white portion are lower than the desired gradation value Lt; conversely, in the case where the zoom position of a zoom lens is set at the telephoto end, gradation values of pixels in the white portion will be higher than the desired gradation value Lt. In either case, the average value of gradation values of the white portion cannot be maintained at the desired gradation value Lt.
Accordingly, where the average value of gradation values of the white portion in a imaged image deviates from the desired gradation value Lt due to change in the zoom position of the zoom lens, when subsequently the imaged image is analyzed and various adjustments are made automatically on the basis of the analysis results as mentioned previously, there resulted the problem of inability to make proper settings, depending on the particulars of the adjustment.
This problem is not limited to cases where the adjustment pattern image is entirely white, but can also occur in cases of specific color other than white (such as green, for example), or on cases where some of the image is white rather than all of it.